This invention relates to roller swaging of tubing, such as for attaching hydraulic fitting sleeves to high strength, thin wall, aerospace hydraulic tubing.
Roller swaging of tubing for attachment of hydraulic fittings is a common practice in the aerospace industry. In roller swaging, the end of an expander assembly is inserted into the tube to be swaged. At the time of insertion, a sleeve to be swaged onto the tube is loosely in place on the tube. The end of the expander assembly includes a plurality of rollers that are free to move radially toward and away from the longitudinal axis of the expander assembly. A rotating tapered mandrel is moved along the axis of the expander assembly, frictionally engaging the rollers and forcing the rollers against the inner wall of the tube. The mandrel continues to rotate and advance, causing the rollers to expand the tube, forcing tube material to flow into grooves in the sleeve, effecting a strong sealed connection between the tube and the sleeve.
Roller swaging operations in the prior art exhibit one or more of the following disadvantages:
1. Poor control of the amount of swaging. Insufficient swaging can cause failure of the joint between the tube and the sleeve; too much swaging can cause the tube to crack. Either condition can result in leakage of an installed fitting. When titanium tubing is used, especially tight control of the amount of swaging is required to obtain acceptable quality.
2. Short tool life. Rapid wear of the swaging rollers is experienced after swaging progresses beyond a desired amount. This disadvantage is related to control of the amount of swaging, discussed above. Undesireable wear also occurs when there is axially sliding contact between the rollers and the mandrel during swaging. Most roller swaging machines of the prior art now provide a slightly skewed roller orientation producing self-feeding of the tapered mandrel without axial sliding against the rollers. However, some roller swaging machines continue rotation of the mandrel after preventing further advancement at the end of swaging. This produces axially sliding contact between the rollers and the mandrel when swaging forces are greatest, producing rapid wear of the rollers and the tapered mandrel.
3. Low speed operation. Manual adjustments and operations are time consuming and therefore expensive. When titanium tubing is used, a slow rate of swaging is required to prevent the tubing from springing back to a smaller diameter following completion of the swage, which can result in failure of the joint.
One method of roller swaging used in the past was to swage the tube to a selected inner diameter by inserting the rotating tapered mandrel a selected distance into the expander assembly. By advancing the mandrel a selected distance into the expander, the rollers are moved outwardly to a known diameter, which in this case is the selected inner tube diameter. It has been found, however, that when swages are made to a selected inner tube diameter, the strength of the swaged connection is not as consistent as desired. This lack of consistency is due to variations in tube wall thickness and diameter, and to a lesser extent, sleeve wall thickness and diameter.
Another method of roller swaging used in the past is to base completion of the swage on a selected torque value required to turn the expander assembly. Using torque as a basis for determining when the swage is complete has been found to provide more uniformly strong swaged joints than are provided when such swaging is based on a selected inner tube diameter.
Devices used in the prior art for terminating roller swaging at a selcted torque value exhibit one or more of the following disadvantages:
1. Individual, interchangeable, pre-set torque limiters are awkward to exchange and store, and impractical to adjust;
2. Torque sensing by measurement of electrical current in a mandrel drive motor can be inaccurate;
3. Torque limiting based on balancing a gimballed motor housing against an adjustable bias such as a spring requires time-consuming manual adjustment of the bias to effect a change in the limiting torque;
4. Torque limiting by manual use of a torque wrench driving the mandrel is too slow for economical production and subject to operator-induced variation; and
5. Non-uniform swaging results from failure to compensate for the torque required to turn the mandrel before swaging actually begins.
Thus there is a need for a roller swaging machine which closely controls the amount of swaging, has long tool life, swages at high speed, is easy to use, and is automatic in operation.